fix(web): retire session archive UI and API, restore archived rows

Remove archive/restore from the session menu and drop archived=1 listing. PATCH still accepts archived_at from old bundles then ignores it. A forward migration clears archived_at without bumping updated_at. Delete stays unchanged.

frontend/src/components/app-sidebar.tsx

@@ -218,7 +218,6 @@ export function AppSidebar({
             onTogglePinned={() => sessionControls.onTogglePinned(session.id)}
             onRename={() => sessionControls.onRename(session.id)}
             onShare={() => sessionControls.onShare(session.id)}
-            onToggleArchived={() => sessionControls.onToggleArchived(session.id)}
             onDelete={() => sessionControls.onDelete(session.id)}
           />
         ) : (
@@ -276,14 +275,14 @@ export function AppSidebar({
 
       <SidebarContent>
         {showExpandedContent ? (
-            <SidebarGroup className="session-nav" aria-label={sessionControls?.showingArchived ? "归档记录" : "最近对话"}>
+            <SidebarGroup className="session-nav" aria-label="最近对话">
               {/* One flat list. It was three stacked sections — a 星盘列表 chip grid
                   and two <details> wrappers for 收藏对话 / 历史对话 — so the nav
                   carried two collapse affordances before the first session row.
                   Pinned sessions keep their priority as the first labelled group,
                   in the same shape as the recency groups under them. */}
               <p className="sidebar-section-label" ref={historyHeadingRef} tabIndex={-1}>
-                {sessionControls?.showingArchived ? "归档记录" : "最近"}
+                最近
               </p>
               <SidebarGroupContent className="sidebar-nested">
                 {favoriteSessions.length === 0 && historySessions.length === 0 ? (

frontend/src/components/sidebar-session-row.tsx

@@ -2,8 +2,6 @@
 
 import { Menu } from "@base-ui/react/menu";
 import {
-  Archive,
-  ArchiveRestore,
   MoreHorizontal,
   Pencil,
   Share2,
@@ -22,23 +20,18 @@ export type SidebarSession = {
   readonly title: string;
   readonly subtitle?: string | null;
   readonly pinned: boolean;
-  readonly archived: boolean;
   readonly updatedAt: number;
 };
 
 export type SidebarSessionControls = {
-  readonly archivedCount: number;
-  readonly showingArchived: boolean;
   readonly hasMore?: boolean;
   readonly onLoadMore?: () => void;
   readonly menuSessionId: string | null;
   readonly disabled: boolean;
-  readonly onToggleArchivedView: () => void;
   readonly onMenuSessionChange: (sessionId: string | null) => void;
   readonly onTogglePinned: (sessionId: string) => void;
   readonly onRename: (sessionId: string) => void;
   readonly onShare: (sessionId: string) => void;
-  readonly onToggleArchived: (sessionId: string) => void;
   readonly onDelete: (sessionId: string) => void;
 };
 
@@ -60,7 +53,6 @@ type SidebarSessionRowInteractiveProps = SidebarSessionRowCommonProps & {
   readonly onTogglePinned: () => void;
   readonly onRename: () => void;
   readonly onShare: () => void;
-  readonly onToggleArchived: () => void;
   readonly onDelete: () => void;
 };
 
@@ -122,7 +114,6 @@ export const SidebarSessionRow = forwardRef<HTMLButtonElement, SidebarSessionRow
     onTogglePinned,
     onRename,
     onShare,
-    onToggleArchived,
     onDelete,
   } = props;
 
@@ -173,10 +164,6 @@ export const SidebarSessionRow = forwardRef<HTMLButtonElement, SidebarSessionRow
             </Menu.Item>
             <Menu.Item className="session-action-item" onClick={onRename}><Pencil aria-hidden="true" /><span>重命名</span></Menu.Item>
             <Menu.Item className="session-action-item" onClick={onShare}><Share2 aria-hidden="true" /><span>转发</span></Menu.Item>
-            <Menu.Item className="session-action-item" onClick={onToggleArchived}>
-              {session.archived ? <ArchiveRestore aria-hidden="true" /> : <Archive aria-hidden="true" />}
-              <span>{session.archived ? "恢复" : "归档"}</span>
-            </Menu.Item>
             <Menu.Separator className="session-actions-separator" />
             <Menu.Item className="session-action-item session-action-danger" onClick={onDelete}><Trash2 aria-hidden="true" /><span>删除</span></Menu.Item>
           </Menu.Popup>